Water is among the most critical natural resources, vital for humans and ecosystems alike. Although covering 70% of the Earth’s surface, only 2.5% is freshwater, mostly concentrated in rivers, lakes, and groundwater. Freshwater ecosystems support essential services like drinking water, agriculture, and energy production, but are increasingly threatened by scarcity and pollution. These ecosystems host 6% of global biodiversity and are highly vulnerable to environmental changes caused by climate change and human activities. Diatoms (Bacillariophyta), key microorganisms in freshwater ecosystems, are sensitive bioindicators of water quality, making their study essential for monitoring ecological changes. This thesis investigates the biodiversity of epiphytic diatoms in the Tigris and Euphrates rivers, a critical freshwater habitat in Iraq. Using an integrative approach that combines both morphological and molecular analyses, the research provides a detailed assessment of diatom diversity. Epiphytic samples were collected from six sites along the Tigris and Euphrates rivers in northern Basra, southern Iraq, during both the dry and rainy seasons of 2019 and 2020. This project was funded by the Heinrich Böll Foundation. The morphological examination identified 284 taxa, representing the largest diatom diversity reported in the region, while metabarcoding (18SV4) revealed 1454 amplicon sequence variants (ASVs) across 108 species. A clear seasonal effect was observed in the composition of the diatom community based on morphological analysis, whereas the 18SV4 analyses did not show such a seasonal effect in community composition for the same samples. This discrepancy is likely due to the incomplete taxonomic reference libraries for molecular data at the studied sites, presenting a challenge for the molecular approach. Nonetheless, the combination of morphological and molecular methods improved the detection and identification of diatom assemblages, underscoring the importance of using complementary approaches to fully understand the region's biodiversity. Additionally, a new diatom species, Prestauroneis furatensis, was described in the Euphrates River. This species, found only in summer samples, was identified through light and scanning electron microscopy. Detailed analysis of the river’s physical and chemical parameters revealed that P. furatensis thrives under elevated conductivity conditions. The discovery contributes to the growing understanding of diatom biodiversity in the Tigris-Euphrates basin. To further verify the stability of diatom characteristics, both morphologically and genetically, sixteen monoclonal pennate diatom strains, identified as 10 species in six genera, were cultivated for up to 18 or 24 months. Throughout this period, molecular data for the 18SV4, rbcL, and psbA markers remained stable across all taxa, whereas morphological features, particularly valve length and shape, showed significant variability. This research emphasizes the importance of cultivation in taxonomic studies, noting that a limited time period and optimal cultivation conditions are essential to avoid the production of teratological frustules. It also underscores the need for both molecular and morphological data to achieve accurate species identification Overall, this thesis provides detailed insights into the diversity and ecology of diatoms in the Tigris and Euphrates rivers. The results of this study emphasize the complementarity of both morphological and molecular approaches. This integrative approach has contributed to a deeper understanding of the biodiversity of epiphytic diatoms in the freshwater of the Tigris and Euphrates rivers. The findings also highlight the increasing salinization tendencies in these rivers, driven by reduced freshwater flow and the intrusion of seawater from the Persian Gulf. These processes threaten the ecological balance and biodiversity of these critical freshwater habitats, exacerbating water scarcity and quality issues in the region. This work lays the foundation for future conservation and monitoring efforts aimed at mitigating the impacts of salinization and preserving the aquatic ecosystems of the Tigris and Euphrates rivers and the broader region.
